Angels’ share challenge

Reinhard Meusinger
0
) NMR Abteilung, FB Chemie,
Technische Universitt Darmstadt
, Petersenstr. 22, 64287 Darmstadt,
Germany
We would like to invite you to participate in the Analytical Challenge, a series of puzzles to entertain and challenge our readers. This special feature of Analytical and Bioanalytical Chemistry (ABC) has established itself as a truly unique quiz series, with a new scientific puzzle published every other month. Readers can access the complete collection of published problems with their solutions on the ABC homepage at http://www.springer.com/abc. Test your knowledge and tease your wits in diverse areas of analytical and bioanalytical chemistry by viewing this collection. In this challenge, flavorants are the topic. And please note that there is a prize to be won (a Springer book of your choice up to a value of 100). Please read on
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Meet the angels share challenge
The small portion of wine or distilled spirit that is lost during
long aging in wood barrels is known as the Angels share (if
this nip is larger than two percent per year, the alertness of
the tax collectors will awaken). Many flavoring compounds
are extracted from the wood of the barrels during this time. In
fact, several hundred flavorants have been detected in these
complex beverages, including a gamut of alcohols, carbonyl
compounds, carboxylic acids and their esters, terpenes,
nitrogen-containing and sulfur-containing compounds,
tannins and other polyphenolic compounds, and
oxygencontaining heterocyclic compounds [1].
In this challenge we are looking for such a compound
which occurs in whiskies, cognac, rum, and also old wine.
The complexity of this subject is grand, which is why this
challenge will be limited to whisky.
The flavor and color of whisky arises due to three
reasons: the distillation, the cask maturing, and
additives. During distillation, the flavor develops in part
because of the presence of fermentation products, for
example acetals, ketones, esters or aldehydes, and higher
alcohols. Most of these compounds contribute to the hangover
(veisalgia) and will not be given closer attention here. The
additional flavor and color of whisky depends strongly on
local regulations. For instance, a Scotch whisky may contain
no additives other than the caramel coloring (E150a). Last, the
presence of our flavorant in whisky is because of the aging
process.
Laws in several jurisdictions require that whisk(e)y
must be aged in wood barrels. Similar regulations exist
for brandy, sherry, and cognac. In addition to the length
of aging, the volume of the barrels and their storage
location, the type of wood and its provenance also play
an important role in the quality of the end product. The
type of the wood used for barrels will be kept a secret
in this challenge. This tree is a symbol of strength and
endurance and a national tree of many countries. In
Greek mythology it was sacred to Zeus and in Norse
mythology it was sacred to Thor, and a legend goes that
the Christianization of Germany was marked by the
felling of this sacred tree by an Anglo-Saxon missionary
in 723. Today, both the common name and the botanical
(Latin) name of this tree features in the trivial names of
our compound.
The barrels for aging alcoholic beverages are made
from European and American trees. The choice between
these two kinds of tree is especially important for wine
producers. For maturing of whisk(e)y, different rules are
prescribed by law. As an example, the straight
whiskey must be stored in the United States for at least two
years in new, charred wood containers; Bottled in
Bond Bourbon whiskey liquor must age for four years
whereas all Scotch whisky must be aged in wood
barrels for at least three years and one day [2]. Therefore,
it seems that only the fanciers of fresh distilled
whisk(e)y, named new spirit, moonshine, or poitin
underestimate the value of our flavor compound. All
other consumers enjoy spirits matured in wood casks.
Misleadingly, the maturing of Scotch whisky in a new
cask does not mean the use of a fresh unused cask. In
Scotland, the first fill typically describes maturing in
an American cask formerly used to mature bourbon
whiskey. Recently, a new trend, the so-called finishing
has gained in popularity with the product known as
double matured or wood-finished. Here, the spirit
spends further time in a second cask, usually one that
has been used to mature fortified wine, sherry, port
wine, Madeira, or even standard wines. However, this
may still not be the end of the life of a wood cask. For
example, the pepper mash used to make Tabasco sauce
is aged for three years in used whiskey barrels (Fig. 1).
From the scientific literature, it appears that our
compound was first reported at the end of the 1960s. For
convenience, it was first christened after the peak
number in the gas chromatogram of red wine flavorants.
Shortly after the first description it was found that four
possible stereoisomers can exist, and in the aftermath a
few authors incorrectly assigned the naturally occurring
isomer. The pure stereoisomers were obtained on a
preparative scale in the mid-1980s and their sensory
properties can now be analyzed with greater accuracy.
However, only two diastereomers will be of interest in
this challenge. Both of these diastereomers are found in
spirits that have been aged in wood containers and their
odor is described as weedy, hay, celery, spicy
for one of them, and sweet, cinnamon, fatty for the
other. The organoleptic description of the mixture of both
diastereomers is coconut and green.
Although the first report of the compound isolated
it in amounts so small that only a relatively poor
infrared spectrum could be obtained, here the reader
Fig. 1 Part of the filling station in the whisky museum in the Dallas Dhu
distillery in Scotland. The last barrel was filled here with a single malt
Scotch whisky on March 16, 1983 and in 1988 it was re-opened to the
public (photo R. Meusinger)
has the luxury set of spectra for structural analysis of
this flavorant. The infrared spectrum (Fig. 2), the
electron-impact mass spectrum (Fig. 3), and the
NMR spectra (Figs. 4 and 5) were obtained from a
synthetic mixture of the two diastereomers (courtesy
of Dr Hans-Georg Schmarr, Head of Wine Analysis
and Microbiology, Center for Wine Research, Neustadt/
Weinstrasse, Germany).
Although irrelevant in IR and MS spectra, both
diastereomers are distinguishable in the NMR spectra. All
NMR spectra were obtained in CDCl3. In Fig. 4 the
500-MHz 1H NMR, DEPT-135, and 13C NMR spectra
of a nonequivalent mixture of both diastereomers are
given. In the 1H NMR spectrum the integral value of
the low-field shifted signal of the major component at
3.93 ppm was set to 1.0. The signals with unambiguous
assignment to the major and minor components are
colored red and blue, respectively. The chemical shift
Fig. 2 Infrared spectrum (liquid film)
Fig. 3 Electron-impact mass spectrum. Note the parent peak at m /e 156 and the base peak at m /e 99
Fig. 4 500-MHz 1H NMR (top ), DEPT-135 (middle , CH3 and CH positive and CH2 negative signals), and 13C NMR (bottom) spectra of a synthetic
mixture of both diastereomers of the compound in CDCl3 relative to tetramethylsilane. The major and minor components are colored red and blue,
respectively
Fig. 5 Two-dimensional 1H-correlated COSY (top) and TOCSY spectra
and the 13C heteronuclear single bond (HSQC) and multiple bond
(HMBC) correlation spectra (bottom)
region in the 13C NMR spectrum is divided for the sake
of clarity.
Figure 5 shows a set of two-dimensional NMR spectra. In
addition to the 1H-correlated COSY spectrum the TOCSY
(TOtal Correlated SpectroscopY) spectrum is also given.
Whereas the COSY experiment generally correlates protons
via geminal or vicinal spin coupling, the TOCSY method can,
in principle, give a total correlation of all protons of a chain
with each other. In the pictured spectrum up to nine signals
may be assigned to a single diastereomer; these emanate from
the most low-field shifted 1H NMR signals. The same color
coding was used here as in the one-dimensional spectrum.
Further, the 13C heteronuclear single bond (HSQC) and
multiple bond (HMBC) correlation spectra are shown in the same
figure. Here the direct couplings (1J H-C) and couplings over
two (2J H-C-C) and/or three (3J H-C-C-C) covalent bonds are
indicated. All spectra are truncated for clarity in both dimensions.
For easier interpretation, note the DEPT spectrum in the F1
projection of the HSQC spectrum. In the HMBC spectrum the
cross peaks of the two 1H methyl signals at 0.94 ppm and
1.07 ppm are particularly remarkable, because each indicates
markedly different correlations with one CH2 and two CH
carbons, respectively. The reason for this is the different
stereochemical arrangement of the aliphatic neighboring groups.
Can you identify the compound described in this
challenge? Salud!
We invite our readers to participate in the Analytical
Challenge by solving the puzzle above. Please send the correct
solution to by December 1,
2013. Make sure you enter Angels share challenge in the
subject line of your e-mail. The winner will be notified by
email and his/her name will be published on the Analytical
and Bioanalytical Chemistry website at http://www.springer.
com/abc and in the journal (volume 406/issue 7) where the
readers will find the solution and a short explanation.
The next Analytical Challenge will be published in 406/1,
January 2013. If you have enjoyed solving this Analytical
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